This invention relates to a variable displacement, pressure compensated pump. More specifically, it relates to a variable displacement, pressure compensated pump in which a servo mechanism adjusts the pressure setting of the pressure compensator mechanism.
A variable displacement, pressure compensated pump can be used in a hydraulic system to provide driving fluid for a plurality of hydraulic actuators. In such a system, the variable displacement pump is driven by a prime mover such as an electric motor. The pump draws low pressure fluid into an inlet port from a reservoir and delivers fluid under pressure from its outlet port to flow control valves which operate the hydraulic actuators. The function of the pressure compensator is to maintain the pressure of the fluid in the outlet port of the pump at a constant set pressure. The compensator responds to changes in the pressure of the outlet fluid by increasing pump displacement when the pressure of the outlet fluid falls below the pressure setting of the pressure compensator and by decreasing pump displacement when the pressure of the fluid in the outlet exceeds the pressure setting of the pressure compensator.
In some hydraulic systems, such as those used on an aircraft, the pressure setting of the pressure compensator is set high enough to ensure that there is adequate pressure fluid for all of the hydraulic actuators which may operate at any one time. The pressure setting is maintained at this high level even though multiple hydraulic actuators may operate simultaneously very infrequently (such as during takeoff or landing of the aircraft) and outlet pressure fluid at the high pressure setting is required during only a small percentage of the time the hydraulic system is operating. The pressure setting of the pressure compensator is set at this high level because it is difficult to change the setting of a pressure compensator during normal system operation.
One problem with having to set the pressure compensator setting of a pump at the high level required for the maximum anticipated load by the hydraulic actuators, i.e., the worst possible case, is that in all hydraulic systems there is some leakage of fluid past pistons, spools and other internal mechanisms, and the amount of fluid leakage increases with pressure. Leakage must be made up through a pump with increased pumping capacity which requires a larger prime mover. This increases the weight of the unit which is particularly undesirable in an aircraft. An additional problem with having to maintain a high pressure compensator setting is the power required to drive a pump increases exponentially with pressure. As the required amount of power increases, fuel consumption increases and the amount of waste heat from the prime mover which must be dissipated also increases. A further problem with having to maintain a high pressure compensator setting is that in most instances the power required to operate a hydraulic actuator is much less than the capacity available. Consequently, when a flow control valve is operated to supply fluid to a hydraulic actuator, energy is lost through throttling of the high pressure fluid down to the level required by the hydraulic actuator. This results in additional heat which must be dissipated.
It is desirable to provide a variable displacement, pressure compensated pump for a system in which multiple hydraulic actuators are operated in which the pressure setting of the pressure compensator can be adjusted to meet the anticipated load demand of the system, such that the pressure setting of the pressure compensator can be maintained at less than the output pressure required for simultaneous operation of all the hydraulic actuators in the system. The system for adjusting the pressure setting of the pressure compensator must be fail safe, such that in the event of an interruption of electrical power or hydraulic fluid, the pressure setting of the pressure compensator is at the maximum and sufficient to meet the demands of simultaneous operation of all hydraulic actuators in the system.